Q: May I install 480V mercury-vapor lighting fixtures in a large store in a shopping center? Are 480Y/277V slash-rated circuit breakers acceptable as branch circuit overcurrent devices even though the branch circuit voltage is 480?

A: Yes, 210.6(C) permits the use of 480V branch circuits supplied from a 480Y/277V system where the voltage to ground does not exceed 277V. Some of the types of luminaires (lighting fixtures) that are acceptable are listed electric-discharge lamps.

Slash-rated (480Y/277V) circuit breakers can be used on these 480V circuits in a solidly grounded system where the nominal voltage of any conductor to ground does not exceed the lower voltage marking on circuit breaker.

Metal water pipe supplement

Q: The electric service for a small grocery store is grounded to the underground metal water pipe where it enters the building. There are no other electrodes present. Will one ground rod driven near the water pipe and connected to the water pipe satisfy the requirement for a supplementary grounding electrode?

A:Not any more. In the 1999 and previous editions of the NEC, one ground rod was sufficient; now two are required where the resistance to earth of one ground rod is more than 25 ohms. The type of metal, minimum diameter, and length of ground rods is mentioned in 250.52(A)(5). Minimum spacing between electrodes is given in 250.53(B).

Disconnecting multiwire branch circuits

Q: Why does the National Electrical Code only require simultaneous disconnection of the ungrounded conductors of multiwire branch circuits that supply split-wired receptacles in dwelling units? Multiwire branch circuits that supply these devices in commercial occupancies present the same hazard to unsuspecting electricians.

A: The requirement for simultaneous disconnection of the ungrounded conductors of a multiwire branch circuit that supplies a single device or equipment in a dwelling unit is found in 210.4(B). An addition to the 2002 edition of the NEC has this requirement in 210.8(C): “Where more than one branch circuit supplies more than one receptacle on the same yoke, a means to simultaneously disconnect the ungrounded conductors supplying those receptacles shall be provided where the branch circuit originated.”

This requirement, along with the statement in 210.4(A) which reads: “A multiwire branch circuit shall be permitted to be considered as multiple circuits”, results in a requirement that all multiwire branch circuits that supply split-wired receptacles in all occupancies must provide simultaneous disconnection of the ungrounded conductors in the panelboard where they originate.

PVC conduit underground

Q: Does the National Electrical Code permit PVC conduit to be run underground for a feeder to a store where the nonmetallic conduit has to pass under a gasoline dispenser island?

A: Yes, rigid nonmetallic conduit is permitted to pass under a gasoline dispensing island by Exception No. 2 of 514.8. The exception requires burial of at least 2 feet below the surface, and the use of threaded rigid metal conduit or threaded steel intermediate metal conduit for the last 2 feet of underground run to the point that the conduit emerges or to the point that the conduit connects to the above ground raceway.

The installation of rigid nonmetallic conduit must also comply with Article 352—Rigid Nonmetallic Conduit: Type RMC. In particular, the conduit must be suitable for the corrosive conditions encountered and be capable of withstanding continued loading that will occur after installation.

Disconnect for residential gas furnace

Q: When wiring a gas-fired central heating furnace in a dwelling unit, I usually provide a toggle switch at the furnace to satisfy the requirement for a disconnecting means within sight of the blower motor. However, some jurisdictions require a fused disconnect and do not allow a toggle switch. Is a fused disconnect required by the National Electrical Code?

A: The disconnecting means for the motor must comply with Parts III of Article 422 and Part IX of Article 430. For appliances with a motor larger than 1/8 horsepower, a disconnect switch must be located within sight of the appliance or must be capable of being locked in the open position. This requirement appears in 422.31(B).

However, a revision in 420.102(B) now requires a disconnecting means within sight of the motor. The Exception to 430.102(B) does not apply to this installation.

Part (C) of 430.109 recognizes a toggle switch as a motor disconnect under limited conditions. To use a toggle (snap) switch, the motor cannot exceed 2 horsepower and the voltage cannot exceed 300.

These are the requirements: a general use snap switch suitable must have an ampere rating that is at least twice the full load current rating of the motor; a general use snap switch suitable for use only on AC (not a general use AC/DC snap switch) may serve as a motor disconnect provided that the motor full-load current does not exceed 80 percent of the Ampere rating at the switch. Similar rules dealing with the use of toggle switches are found in 404.14(A) and (B).

Part (A) of 422.11 covers branch circuit overcurrent protection. This part says that, “If a protective device rating is marked on an appliance, the branch-circuit overcurrent device rating shall not exceed the protective device rating marked on the appliance.”

Does the branch circuit overcurrent protective device rating in the panelboard exceed the value marked on the appliance? If the overcurrent protective device in the panelboard is rated 20A and the nameplate on the furnace calls for a 15A overcurrent protective device, this could be the reason for the inspector’s request for a fused disconnect at the furnace rather than a toggle switch.

Fire alarm wiring

Q: I am installing a completely new fire alarm system that will replace the existing fire alarm system in an office building. Do I have to remove all of the old fire alarm wiring?

A: Where abandoned cable can be removed, it is not permitted to remain. These are the words in 760.3(A): “The accessible portion of abandoned fire alarm cables shall not be permitted to remain.” Abandoned cable is defined in 760.2 as “Installed fire alarm cable that is not terminated at equipment other than a connector and not identified for future use with a tag.”

Other places in Article 760 also require the removal of abandoned fire alarm cables. Some judgment has to be used in deciding which cables can be removed without damaging other low-voltage cables that are used for other communication systems. Part 700.3(A) does not require the removal of all abandoned fire alarm cables. It only requires removal of the accessible portions of abandoned fire alarm cable.

Sizing services and feeders for apartment buildings

Q: Our electrical contracting firm, along with another electric contractor (joint venture), have been wiring some six-unit apartment buildings. The other contractor has been installing 125A panels in each unit. I disagree with him and think that the minimum size should be 150A. We also usually only load our panels to 80 percent of the main circuit breaker size in single-family homes. I believe the 125A panels are below the minimum. Maybe you can tell me who is right and who is wrong.

Also can we use the demand factors shown in Table 220.32 for each six-unit building? We were going to use parallel 250 kcmil aluminum conductors for the service. We have bid these units using the 125A panel size. But I would like to know if this size is correct just to ease my mind.

A: Using the figures provided and the demand factors permitted by Table 220.11 for the load calculated by the square foot area, the two small appliance branch circuits, and the laundry circuit results in a calculated load of 102A for a 120/240V, single-phase service for each apartment.

I notice that there is no air conditioning load in the list provided, but according to my calculations, air conditioning could be added provided that 125 percent of the largest motor load, plus the full load current of evaporator fan and condenser fan motors do not exceed 23A.

Table 220.32 may be used to size the service for each building, but you must follow the rules in 220.32. Each appliance that is fastened in place or connected by permanent wiring methods must be put into the calculations at nameplate current ratings. It is possible that the use of this Table will result in a smaller service than obtained by using the standard method outlined in Part II of Article 220. EC

FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.254.2132.